Magnetic Resonance Imaging (MRI) compatibility has become an important feature for medical devices as the use of MRI increases and becomes more commonplace. Over the past three decades, MRI has become a valuable medical tool due to its offer of better soft tissue contrast and tissue chemistry, which are not currently obtainable with standard x-ray fluoroscopic imaging, spiral CT scanning and/or ultrasound. With increased computing power and higher strength magnetic fields, today's MRI equipment provides better signal to noise ratios (SNR) and offers the medical community more detailed imaging capabilities. In the mid-1990's, most clinical MRI equipment was based on a 0.5 Tesla field. By increasing field strength to 1.5 T, and then to 3.0 T, the SNR and image quality has improved without inconvenience to the patient or physician. As a result, MRI imaging has become more common in the neurology and internal medicine fields.
The MRI technique utilizes intense magnetic fields. Nonetheless, some biocompatible devices are constructed from metals or metal alloys that are not MRI compatible. MRI incompatibility arises from an interaction with the magnetic field, resulting in a physical force and/or heating of the metal, and/or distortion of the magnetic resonance (MR) image. These incompatibilities may be life threatening. An extreme example would be the reaction of the MRI magnetic field with a ferromagnetic implant, causing the implant to be dislodged from the therapeutic tissue location, resulting in injury, or death. A no less life threatening example of incompatibility would be the malfunctions of a cardiac rhythm device due to incompatible part(s). Alternatively, an incompatible material may have no noticeable physical reaction, typical of many paramagnetic and diamagnetic materials, but may cause image distortion (e.g. image ghosting, image artifact). The image artifact, or “ghost,” can be considered an obstruction, and may limit the capability of the MRI to image the surrounding area and thereby reduce the physician's ability to examine critical features. These, so called ghost image effects, can be greater than ten times the size of the object causing the distortion. The less a metal interacts with a magnetic field, the more compatible it will be for an MRI application. In general a material that is not compatible due to magnetic force will not be compatible for image clarity. The field of research shows that device heating of metals in an MRI is largely due to device geometry. Also there are indications that a material's electrical properties may impact image distortion. To complicate the matter further, not all MRI compatible metals or metal alloys are biocompatible, in that the implant or medical device causes an adverse bodily or localized reaction in use.
Some have sought to improve the MRI compatibility of the materials used to construct devices, including the improvement of the MRI compatibility of palladium based metal alloys. In U.S. Pat. No. 7,087,077, filed Mar. 27, 2002, and entitled Biomedical Aid or Implant; and U.S. Patent Application Publication No. 2005/0121120, filed Jan. 14, 2005, and entitled Biomedical Aid or Implant (“the '120 publication”), the use of specific ratios of palladium, gold, and platinum is disclosed, along with additional dopants to improve the alloys imaging characteristics in an MRI. U.S. Patent Application Publication No. 2007/0280850, filed Sep. 23, 2005, and entitled MRI Compatible Devices (“the '850 publication”), discloses a generic combination of “precious” metals and “refractory” metals, to achieve particular properties, such that the magnetic susceptibility of the metal or alloy is greater than, or less than, the magnetic susceptibility of the base metal. The reference sets an upper bound of 3×10−4 (cgs), volumetric magnetic susceptibility, which is nearly five times greater than pure palladium's magnetic susceptibility of 6.1×10−5. U.S. Patent Application Publication No. 2007/0162108, filed Dec. 13, 2003, and entitled Implantable Medical Device Using Palladium (“the '108 publication”), provides a similar list of alloying elements. Additionally, U.S. Pat. No. 7,128,757, issued Oct. 31, 2006, and entitled Radiopaque and MRI Compatible Nitinol Alloys for Medical Devices (“the '757 patent”), discloses alloys containing palladium, but seeks to “maintain” the MRI compatibility of the alloy. For example, the susceptibility of Nitinol is listed as 1.9×10−5, and the susceptibility of a Ni—Ti—Pt alloy is provided as 1.33×10−5.
In view of the aforementioned references, although some have addressed the general issue of MRI compatibility of medical aids and implants in mechanical, geometrical, or electrical device applications using generic materials, none have dealt with technology needed to formulate alloys with ultra-low magnetic susceptibilities. Accordingly, there is a need for biocompatible medical devices or aids that are MRI compatible, which have an ultra-low magnetic susceptibility.